Applicant""s invention relates to electrical telecommunication, and more particularly to wireless communication systems, such as cellular and satellite radio systems, for various modes of operation (analog, digital, dual mode, etc.), and for access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrid FDMA/TDMA/CDMA.
In a TDMA cellular radio telephone system, each radio channel is divided into a series of time slots, each of which contains a burst of information from a data source, e.g., a digitally encoded portion of a voice conversation or digital control information. The time slots are grouped into successive frames that each have a predetermined duration. Successive time slots assigned to the same user, which are usually not consecutive time slots on the radio carrier, constitute a logical channel assigned to the user. As described in more detail below, such logical channels are provided for communicating control signals and for voice and data signals.
It can be seen that a TDMA cellular system operates in a buffer-and-burst, or discontinuous-transmission, mode: each terminal transmits (and receives) only during its assigned time slots or frames. Therefore, portions of the terminal, or mobile station (MS), which may be battery-powered, can be switched off, or xe2x80x9csleepxe2x80x9d, to save power during the time slots when it is neither transmitting nor receiving. During assigned slots, a MS awakes and monitors the control channel for paging messages addressed to it.
For example, when an ordinary telephone (land-line) subscriber calls a mobile subscriber, the call is directed from the public switched telephone network (PSTN) to a mobile switching center (MSC) that analyzes the dialed number. If the dialed number is validated, the MSC requests some or all of a number of radio base stations (BSs) to page the called MS by transmitting over their respective control channels page messages that contain information, such as a mobile identification number (MIN), that identifies the called MS.
Each idle MS receiving a paging message compares the received identifying information with its own stored information. The MS having the matching identifying information transmits a page response over the particular control channel to the BS, which forwards the page response to the MSC. Upon receiving the page response, the MSC selects a traffic channel available to the BS that received the page response, switches on a corresponding radio transceiver in that BS, and causes that BS to send a message via the control channel to the called MS that instructs the called MS to tune to the selected voice or traffic channel. A through-connection for the call is established once the MS has tuned to the selected traffic channel.
A fully digitized type of cellular network uses logical channels for communicating digital voice or data as well as control information. One such digital cellular standard is defined by the TIA/EIA-136 standard, which is promulgated by the Telecommunications Industry Association and the Electronic Industries Association and is being adopted rapidly throughout the world. A cellular network implementing the TIA/EIA-136 standard transmits control information over digital control channels (DCCHs). A forward or downlink (BS to MS) DCCH includes successive repetitions of an ordered sequence of logical channels that makes up what is called a superframe. Such networks are described in U.S. Pat. No. 5,604,744 to Andersson et al. for xe2x80x9cDigital Control Channels Having Logical Channels for Multiple Access Radiocommunicationxe2x80x9d, which is incorporated in this application by reference.
FIG. 1 shows a general example of a forward DCCH configured as a succession of time slots 1, 2, . . . , N, . . . included in the consecutive time slots 1, 2, . . . sent on a carrier signal. The DCCH slots may be defined on a radio-frequency carrier signal such as that specified by TIA/EIA-136, and may consist, as seen in FIG. 1 for example, of every n-th slot in a series of consecutive slots that can be organized in TDMA blocks and frames. Each slot may have a duration of 6.67 milliseconds (ms), which is also the duration of a traffic-channel slot according to the TIA/EIA-136 standard. As shown in FIG. 1, the DCCH slots may themselves be organized into superframes (SF), and a superframe according to the standard may include thirty-two DCCH slots and have a duration of 640 ms.
Each superframe typically includes an ordered sequence of a number of logical channels that carry different kinds of information. One or more DCCH slots may be allocated to each logical channel in the superframe. The downlink superframe depicted in FIG. 1 includes at least three logical channels: a broadcast control channel (BCCH) that includes six successive DCCH slots for overhead messages; a paging channel (PCH) that includes one slot for paging messages; and an access response channel (ARCH) that includes one slot for channel assignment and other messages. Other channels may be included in the exemplary superframe of FIG. 1, such as additional PCHs or other channels. As described in more detail below, other organizations of channels in a superframe are possible.
The superframes of a forward DCCH are advantageously organized into hyperframes, with one arrangement being depicted by FIG. 2 in accordance with the TIA/EIA-136 standard. Each hyperframe comprises two superframes, one of which is usually designated the primary superframe and the other of which is usually designated the secondary superframe. A complete hyperframe (hyperframe 0) and a successive partial hyperframe (hyperframe 1) are shown in FIG. 2. Each superframe comprises time slots that are organized into the following logical channels: a fast BCCH (F-BCCH), an extended BCCH (E-BCCH), a short message service BCCH (S-BCCH), and a SPACH. The SPACH typically comprises a short message service channel (SMSCH), a plurality of PCHs, and an ARCH, although any combination of these can make up a given SPACH frame. As noted above, each superframe may also include slots for other logical channels.
According to the TIA/EIA-136 standard, each superframe includes a complete set of F-BCCH information, which is system-related information such as the structure of the DCCH that a MS uses for accessing and maintaining communication with the BSs. This is described in Section 5.1.1, for example, of TIA/EIA Pub. No. SP-4027-123-A (Nov. 20, 1998). Also, every PCH in a primary superframe is repeated in the corresponding secondary superframe. Each MS is hashed to a DCCH in a cell based on a number of parameters, including portions of its user group identity or permanent mobile station identity, the number of DCCHs in the cell, and the number of slots allocated to the DCCHs. This process is described in TIA/EIA Pub. No. SP-4027-121-A, Section 8.1 (Nov. 20, 1998), which is incorporated in this application by reference.
Besides supporting sleep modes for MSs, digital control and traffic channels facilitate optimization of system capacity and support hierarchical cell structures, i.e., structures of macrocells, microcells, picocells, etc. The term xe2x80x9cmacrocellxe2x80x9d generally refers to a cell having a size comparable to the sizes of cells in a conventional cellular telephone system (e.g., a radius of at least about 1 kilometer), and the terms xe2x80x9cmicrocellxe2x80x9d and xe2x80x9cpicocellxe2x80x9d generally refer to progressively smaller cells. For example, a microcell might cover a public indoor or outdoor area, e.g., a convention center or a busy street, and a picocell might cover an office corridor or a floor of a high-rise building. From a radio coverage perspective, macrocells, microcells, and picocells may be distinct from one another or may overlap one another to handle different traffic patterns or radio environments.
FIG. 3 is an exemplary hierarchical, or multi-layered, cellular system. An umbrella macrocell 10 represented by a hexagonal shape makes up an overlying cellular structure. Each umbrella cell may contain an underlying microcell structure. The umbrella cell 10 includes microcell 20 represented by the area enclosed within the dotted line and microcell 30 represented by the area enclosed within the dashed line corresponding to areas along city streets, and picocells 40, 50, and 60, which cover individual floors of a building. The intersection of the two city streets covered by the microcells 20 and 30 may be an area of dense traffic concentration, and thus might represent a hot spot.
FIG. 4 represents a block diagram of an exemplary cellular mobile radio telephone system, including an exemplary BS 110 and MS 120. The BS 110 includes a control and processing unit 130, which is connected to an MSC 140, which in turn is connected to a PSTN (not shown). General aspects of such cellular radio telephone systems are known in the art, as described for example by U.S. Pat. No. 5,175,867 to Wejke et al. for xe2x80x9cNeighbor-Assisted Handoff in a Cellular Communication Systemxe2x80x9d and U.S. Pat. No. 5,353,332 to Raith et al. for xe2x80x9cMethod and Apparatus for Communication Control in a Radiotelephone Systemxe2x80x9d. Both of these patents are incorporated in this application by reference.
The BS 110 handles a plurality of voice channels through one or more voice channel transceivers 150, which are controlled by the control and processing unit 130. Also, each BS includes at least one control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the BS or cell to MSs locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with DCCHs and other channels.
The MS 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170, and the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the mobile station to lock on to, and determines on which cell the mobile should lock.
Under the TIA/EIA-136 standard, PCHs belonging to particular DCCHs are allocated to MSs based on a number of parameters, including for example each MS""s user group identity and its permanent mobile station identity, which can be a MIN. This process is described in Sections 8 and 8.2 of TIA/EIA Pub. No. SP-4027-121-A (Nov. 20, 1998), which are incorporated in this application by reference. In general, the page messages in a TIA/EIA-136 system may be scattered over many PCHs of a DCCH, and depending on the configuration of the network, a cell may further scatter its PCH channels over many DCCHs.
One consequence of MS sleep mode is that page messages can only be sent in assigned PCHs, which allows for potential congestion of the PCHs. This consequence is built into the TIA/EIA-136 definition of PCH functionality and has been observed in practice. Some network operators have complained that they see a higher number of rejected page messages than they would expect on their DCCHs, which should have higher nominal capacity than analog control channels.
It is an object of Applicant""s invention to address this and other aspects of current telecommunication systems by providing methods of reducing deleted messages in a transmitter that sends messages organized in frames, superframes, and logical channels. At least one of the channels includes structure messages that describe the organization of the frames and logical channels and a number of frames to be read by a receiver, including a number of additional frames that should be read when at least one displaced message exists.
In one aspect of Applicant""s invention, a method includes the steps of finding messages to be displaced; placing found messages in a displacement queue, and assigning messages to a frame according to a predetermined order of logical channels, including any displaced messages that fit in the frame. A message is displaced if it will not be sent in a frame of at least one logical channel and will timeout before it can next be sent, and if a message is placed in the displacement queue, then an information element is set in a message, indicating that the number of additional frames is to be read.
The displacement queue may have a length that is based on a maximum number of messages that can be sent in the logical channel and the number of additional frames to be read, and the length may be three and the number of additional frames to be read may be one. Also, the predetermined order of logical channels may be undisplaced messages, other messages, and displaced messages. In addition, the method may further include the steps of removing any displaced messages included in the frame from the displacement queue, and deleting any displaced messages left in the displacement queue and sending respective notification messages for deleted displaced messages. A receiver reads the number of additional frames in response to the set information element.
In another aspect of Applicant""s invention, an apparatus for reducing deleted messages includes a device for finding messages to be displaced; a displacement queue, in which found messages to be displaced are stored; a device for setting, if a message is placed in the displacement queue, an information element in a message indicating that the number of additional frames is to be read; and a device for assigning messages to a frame according to a predetermined order of logical channels, including any displaced messages that fit in the frame. A message is displaced if it will not be sent in a frame of at least one logical channel and will timeout before it can next be sent.
The displacement queue may have a length that is based on a maximum number of messages that can be sent in the logical channel and the number of additional frames to be read, and the length may be three and the number of additional frames to be read may be one. The predetermined order of logical channels may be undisplaced messages, other messages, and displaced messages. Displaced messages included in the frame may be removed from the displacement queue, and displaced messages left in the displacement queue may be deleted and a respective notification message may be sent for each deleted displaced message.